Better caching for nix-based builds

[choener]

• Caching for nix-based builds, using github CI, is currently not operational
• We have a set of builds that all depend on /exactly the same/ nix builds, thus wasting resources
• Setup caching, either using s.th. like cachix, or internal github caching
• Initial code is available as part of the nix-ci build, but restoring the cache does not work

[choener]

Possibly use our arm-based (build) server to store the nix cache.
(Will evaluate soon, since I need to setup something similar on my personal server, too).